求二叉树的最小深度

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

class Solution {
public:
    int minDepth(TreeNode *root) {
        if(root == NULL ) return NULL;
        if(root->left==NULL && root->right == NULL) return 1;
        else if(root->left == NULL && root->right !=NULL) return minDepth(root->right)+1;
        else if(root->left !=NULL && root->right ==NULL) return minDepth(root->left)+1;
        else if(root->left !=NULL && root->right != NULL) return minDepth(root->left)<=minDepth(root->right)?(minDepth(root->left)+1):(minDepth(root->right)+1);
    } 
};